Field
During single radio voice call continuity procedures, a scenario may arise in which a circuit switched (CS) bearer reservation is successful but a voice media switching from packet switched (PS) to CS performed by a mobile switching center (MSC) server fails.
Description of the Related Art
FIG. 1 illustrates an overall single radio voice call continuity (SRVCC) procedure from an evolved universal terrestrial radio access network (E-UTRAN) to a global system for mobile communications (GSM) edge radio access network (GERAN) without dual transfer mode (DTM) support. Discussion of this procedure may be found in TS 23.216 v 11.2.0, which is incorporated herein by reference in its entirety.
As shown in FIG. 1, at S1, measurement reports can be sent from a user equipment (UE) to a source E-UTRAN. Then, at S2, the source E-UTRAN may make a decision for handover (HO). At S3, the E-UTRAN can send a message to a source mobility management entity (MME) indicating that handover is required. At S4, the source mobility management entity can perform bearer splitting. Then, at S5, the source mobility management entity can send a message to a mobile switching center (MSC) server/mobility gateway (MGW) indicating that packet switched (PS) to circuit switched (CS) is required. The mobile switching center Server/MGW can, at S6, send a message to a target mobile switching center indicating that preparation for handover is required. At S7, the target mobile switching center and target base station server (BSS) exchange a handover request and acknowledgement (ACK).
Then, at S8, the mobile switching center server/MGW can receive a response regarding handover preparation from the target mobile switching center. At S9, the mobile switching center server/MGW can establish a circuit with the target mobile switching center. Then, at S10, session transfer can be initiated by communication with an internet protocol (IP) multimedia subsystem (IMS). At S11, the IP multimedia subsystem can perform session transfer and update the remote end. Then, at S12, the IP multimedia subsystem can release the IP multimedia subsystem access leg.
Meanwhile, at S13, the mobile switching center server/MGW can send a packet switched to circuit switched response to the source mobility management entity, which—in turn—can send a handover command to the source E-UTRAN at S14. At S15, the source E-UTRAN can send a command to the user equipment to hand over from E-UTRAN. Then, at S16, the user equipment can tune to GERAN. This can be followed by, at S16, handover detection, suspension of the E-UTRAN side (S18, S18a, S18b), handover completion (S19), and various release, update, and reallocation steps (S20-S24).
Conventionally, the outcome of S10 has no influence on S13. As soon as the circuit switched radio bearer reservation is successful in S8, then MSS returns PS to CS Resp in S13. This triggers the radio handover procedure from long term evolution (LTE) to circuit switched in S16.
There are some reasons why the IP multimedia subsystem session transfer procedure may fail in S10 and/or S11. One reason is that the home subscriber server (HSS) failed to update the session transfer number for SR-VCC (STN-SR) in the mobility management entity, or for any other reason the session transfer number for single radio voice call continuity in the mobility management entity is incorrect. In R10 with eSRVCC, the session transfer number for single radio voice call continuity can point to an access transfer control function (ATCF) (in a serving public land mobile network (PLMN)) where the session is anchored. The HSS can update the mobility management entity with the session transfer number for single radio voice call continuity pointing to the access transfer control function. If the access transfer control function is changed, for example, if the user equipment is roaming to a new public land mobile network, then the HSS would conventionally need to update the current mobility management entity with a new session transfer number for single radio voice call continuity. The reasons why an HSS update to mobility management entity may fail can include a transient network issue, such as signaling link congestion, bad network configuration, or the like.
FIG. 2 illustrates session initiation protocol (SIP) level aspects of eSRVCC using an access transfer control function. As shown in FIG. 2, after a media path is set up, while there is interaction, at S1, between user equipment, radio access network (RAN), MME/SGSN, and mobile switching center as specified in 3GPP TS 23.216 (which is incorporated herein by reference in its entirety), a session initiation protocol INVITE (S2) can be sent to the access transfer control function. The access transfer control function can send (S3) a configuration message to an access transfer gateway (ATGW) and receive (S4) an acknowledgment of the message. Then, the access transfer control function can send (S5) a response to the session initiation protocol INVITE. Subsequently, a new media path can be set up. The access transfer control function can then send (S6) an access transfer update message to a service centralization and continuity application server (SCC-AS)/serving call session control function (S-CSCF) and receive (S7) a response including session state information (SSI). The access transfer control function can then send (S8) the SSI to the mobile switching center server.
If the session transfer number for single radio voice call continuity update fails, the single radio voice call continuity may or may not work anymore. If the session transfer number for single radio voice call continuity stored in the mobility management entity is the one pointing to a service centralization and continuity application server (SCC-AS) (in a home PLMN (H-PLMN)) then the single radio voice call continuity can continue to work as defined for R8/9 and without eSRVCC functionality. If the session transfer number for single radio voice call continuity pointing to the access transfer control function is from the previous/old visited network, then neither single radio voice call continuity nor eSRVCC can work. The reason for the inability of single radio voice call continuity or eSRVCC to work is because the INVITE from the MSS (S2) is, in such a scenario, sent toward an access transfer control function which has no subscription info for the user. The end result is that the user equipment is handed over to circuit switched side in the MSS, but the access transfer from IP multimedia subsystem to circuit switched domain, which is initiated by the INVITE (S2), is not successful. The MSS can conventionally only release the circuit switched call at this point, and user equipment may return back to LTE. This kind of unrealizable single radio voice call continuity can continue to occur for this user equipment, because the mobility management entity has no idea that its session transfer number for single radio voice call continuity is invalid.
There is no conventional way to handle this situation.